1. Field of the Invention
The invention relates to a generic cleaning medium, i. e., a cleaning composition, for cleaning the components in a printing press, in particular for the cleaning or removal of reusable, imaged lithographic printing forms which have been through one printing process. More particularly, the present invention relates to a cleaning medium for printing forms, which are imaged by means of a laser induced thermal transfer ribbon technique. The invention also relates to concentrates of the cleaning medium and its use in containers for shipping and for application. Furthermore, the present invention also relates to an erasure method for using the cleaning medium of the present invention.
2. Description of the Related Art
European Reference EP-B-0 570 879 discloses a method and a device for repeated erasure of the ink layer from the surface of an imaged printing form as used in offset printing. A solvent free jet of water under pressure is directed at an angle onto the imaged surface by means of an erasing device to clean the imaged surface. The medium, which essentially comprises solvent free water under pressure, can contain abrasive additives such as grit and the like or chemical additives having wax dissolving properties, to increase the removal capacity of the jet.
European Reference EP-B-0 693 371 discloses an erasable printing form together with a method and a device for erasing and regenerating the printing form. The reference teaches that after the printing process the printing ink residue and the imaged layer are first removed by a cleaning medium, for example, wiped off. The cleaning medium is a solvent or a solvent agent mixture, which does not contain solid components. In order to remove the last of the residue of the imaged layer on the printing form, however small this is, the surface is subsequently rubbed mechanically. A cleaning medium containing a polishing agent is used, for example an ordinary plate cleaner, which is generally known to be used in for manual cleaning of printing forms. This plate cleaning agent is then removed afterwards, for example, with water.
The mechanical interaction between the cleaning media and the printing form is by means of a cleaning device, which is provided with a cleaning cloth or non-woven cleaning fabric, and which is pressed from a supply roller (clean roll) over another roller against the printing form and then rolled up onto a winding roller (dirty roll).
European Reference EP-B-0 698 488 discloses a method and device for the manufacture of a printing form, whereby a synthetic resin composition is image transferred from a thermal transfer foil to a rotating printing form cylinder by means of a laser. The material transferred by the thermal transfer foil forms the ink supply layer of the printing form.
In pending German Patent Application No. 199 37478.3, a thermal transfer foil or a thermal transfer ribbon for imaging of lithographic printing forms is disclosed, including a substrate layer to which a donor layer is applied. The substrate layer in this case is composed of at least one polymer substance, preferably PET, which has at least the following properties: mechanical stability at a temperature of 150xc2x0 C. and transmission greater than 70% for a light band of from 700 to 1600 nm. The donor layer includes at least the following components: a substance which can transform the radiation energy of an impacting laser light into heat energy, a polymer which includes acid groups and/or their substitute amide groups (where appropriate) and if necessary, a wetting aid. Preferably the substance to transform radiation energy to heat energy is carbon black. Preferably, the acid groups of the polymer include a styrene/(meth) acrylic acid/(meth)acrylate copolymer and/or their substitute amide groups, where appropriate. Preferably, the working agent is methylethylketone (MEK).
Presently, commercially available plate cleaners or other media for cleaning printing forms or other media for cleaning printing forms for the lithographic printing process, but also for cleaning rubber sheets and other movable soiled parts of the printing machine, either fail to meet, or only partially meet the following essential requirements:
(a) sufficient viscosity;
(b) work safety and hygiene;
(c) non-destructive to printing form;
(d) non-abrasive to printing form;
(e) absorbent cleaning cloth;
(f) complete emulsification of cleanser agent; and
(g) non-deteriorative of lithography printing surfaces.
Deficiencies noted in prior art cleaning compositions regarding the above mentioned requirements include:
(a) Unsuitable viscosity or unsuitable rheologic behavior, e.g. thixotrophy, prevents application of cleaning media to the printing form or to a cleaning cloth to be used for cleaning, without causing difficulties hydraulically and with respect to fluid mechanics.
(b) The known formulations did not conform to all work hygiene and technical safety requirements, in particular in connection with closed printing machines, in which an erasure process is to be performed (e.g., problems include aerosol formation dripping and the like);
(c) Since the erasure process is to be performed inside the printing machine, i.e., without removing the printing form, the known formulations are often chemically too aggressive. For example, solvents have a detrimental effect on synthetic material, rubber and other vulcanized rubber parts. Other aggressive and corrosive influences have also been noted.
(d) The abrasive effect on the printing form exceeds the acceptable tolerance in the area of working pressure of the cleaning device and leads to damage (e.g., scratches, abrasive agent deposits, etc.) on the printing form.
(e) The wetting nature of the cleaning cloth, both during application of the cleaning medium to the printing form and also in the removal of the loosened xe2x80x9cdirtxe2x80x9d (ink residue, imaging material, wetting agent constituents, paper dust, etc.) must be such that the fluid constituents can penetrate the cleaning cloth. This is so as to avoid, for example, the cleaning medium dripping during its application. When removing the loosened xe2x80x9cdirtxe2x80x9d, for example, this is then prevented from reforming on the printing form through the non-woven web.
(f) The cleaner is not completely emulsive in water, such that sufficient transport and rinsing properties are not maintained and ready transport of unused cleaner is not possible.
(g) Certain cleaning media irreversibly deteriorate the wetting properties of the printing surface or alter the printing surface in an unfavorable manner (i.e., background hue) in lithography applications.
The object of the present invention is to overcome the limitations of the prior art by providing a simple cleaning composition that avoids the use of additional stabilizing agents which are not part of the cleaning process, and which can cause possible interference with the subsequent flatbed printing. The cleaning medium of the present invention includes:
(a) a substance which produces a pH of from 1 to 4 in aqueous solution, or a substance which produces a pH of from 10 to 14 in aqueous solution;
(b) dispersible abrasive agents;
(c) surfactants and if required complexing agents;
(d) an organic solvent;
(e) water and if required, further technically necessary additives.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.